President Obama is on his way to California to raise money for Senator Barbara Boxer. She's one of many Democratic incumbents facing tough re-election campaigns this year. With the Democrats control of both houses of Congress at stake, the White House has some difficult decisions to make about how and where to deploy the president.

NPR's national political correspondent Mara Liasson reports.

MARA LIASSON: Whats the highest and best use of a sitting president in an election year when he's not on the ballot himself? Some would argue it's what this president is doing tonight in San Francisco: raising big bucks for Democrats. But President Obama has also been showing up to help Democratic candidates actually ask for votes. Here's a sampling.

(Soundbite of cheering)

President BARACK OBAMA: Thats what Martha Coakley is about.

Thats what Creigh Deeds is committed to.

Let's get Jon Corzine re-elected.

(Soundbite of cheering)

President OBAMA: I love you, and I love Arlen Specter. I appreciate you guys.

LIASSON: If you're keeping score, of course, every one of those four candidates lost - not a good record for presidential coattails.

Another Democrat the president endorsed, Arkansas incumbent Senator Blanche Lambert Lincoln, was forced into a runoff last week. In that case, the White House prudently decided not to have the president campaign for her in person. Arkansas was a state he lost badly in 2008.

Mr. PAUL BEGALA (Democratic Strategist): This president gets 53 percent of the vote; he carried 28 states. And yet there are still some places where it doesn't help as much to send him.

LIASSON: That's Democratic strategist Paul Begala.

Mr. BEGALA: I think that the better strategy is to use that national perch to define the choice. Yes, he can raise money, but it's less showing up and saying I love X, vote for X, than it is framing up the philosophical choice.

LIASSON: Begala has been through this before. The Democratic president he advised, Bill Clinton, tried hard to help his party hang on to its majorities in Congress in 1994 but failed. At this point in that election cycle, Clinton had an approval rating of around 51 percent, a few points higher than President Obama is now. But by Election Day in '94, Clinton's approval was down to around 40 percent, weighing on his party. Still, Begala says, even unpopular presidents can help their party in a midterm.

Mr. BEGALA: That's what Reagan did. In Reagan's midterm, his job approval was 44, measurably lower than Obama's, but he sustained minimal losses because he defined the choice. He spent every major speech trashing liberal economic policies and saying that that was what the Democrats wanted to go back to. And that's what I'd like to see Obama do.

LIASSON: In reverse, that is. And bashing Republican policies of the past is just what the president plans to do on the campaign trail, says his top political adviser, David Axelrod. Axelrod sees November not as a referendum but as a choice.

Mr. DAVID AXELROD (Presidential Adviser): It's an easy race to frame; you have a choice between competing visions. We've got a Republican Party that hasn't really offered anything new. And their basic posture, is let's go back to doing the same things that got us into the crisis in the first place, the same irresponsible policies, the same lack of oversight on Wall Street, the same fiscal policies that brought about these problems.

LIASSON: The president can also rally his party's base by traveling to the right places; in Mr. Obama's case, states where he's still popular. But a president who is very unpopular with certain segments of the electorate can also have the perverse effect of energizing his opponents. The Republican candidate for Senate in Kentucky, Rand Paul, recently told CNN that the GOP was quote: licking our chops at running against President Obama.Obama.

Dr. RAND PAUL (Senatorial Candidate, Kentucky): What I tell to the national Democrats is bring it on and please, please, please bring President Obama to Kentucky. We want him to come and campaign for my opponent. In fact, we'll pay for his plane ticket if President Obama will come down to Kentucky.

LIASSON: But chances are President Obama will not come to Kentucky to campaign. He'll do what he can to defeat Rand Paul and other Republicans from safer spots on the national stage, ones where he has more leverage and control of a bigger megaphone.

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